Warning a following vehicle in the case where there is the possibility of a collision can be considered as a method for avoiding a collision between a host vehicle and the following vehicle.
Japanese Unexamined Patent Application Publication No. 2010-271906A discloses a driving assistance device that prevents the occurrence of driving errors caused by tailgating by a following vehicle. With the device according to Japanese Unexamined Patent Application Publication No. 2010-271906A, it is determined that tailgating is occurring when a state in which an inter-vehicle distance is less than or equal to a predetermined inter-vehicle distance continues for greater than or equal to a predetermined fixed length of time.
However, there are cases where the occurrence of tailgating cannot be correctly determined simply by determining the inter-vehicle distance and the duration. Furthermore, there are other types of dangerous driving aside from tailgating, and there are situations where such dangerous driving cannot be determined by the inter-vehicle distance and the duration. Therefore, reliably detecting unnatural behavior in vehicles and the like in the surroundings of the host vehicle and making a warning is thought to be important with respect to increasing the likelihood of avoiding a collision between a host vehicle and a following vehicle.